The present invention relates to a load control circuit for generating a duty signal based on a triangular wave signal and a reference potential to control a load with the duty signal.
Conventionally, meters, such as a speedometer and a tachometer, are arranged in an instrument panel of a vehicle. Switches, such as an air conditioner switch and a car stereo switch, are arranged on a center cluster. Further, lamps for illuminating the meters and light emitting diodes (LEDs) for illuminating the switches are arranged in the instrument panel and the center cluster. The lamps and LEDs are automatically illuminated when the headlights are turned on. The user of the vehicle operates a brightness adjustment switch, arranged in the instrument panel, to adjust the brightness of the lamps and LEDs.
A rheostat (light adjustment device) is installed in the vehicle to control the illumination of the lamps and LEDs. The rheostat illuminates the lamps and LEDs when the headlights are turned on and further adjusts the brightness of the lamps and LEDs in accordance with the operated amount of the brightness adjustment switch. The rheostat includes a triangular wave generation circuit connected to a capacitor, a waveform shaping circuit connected to the triangular wave generation circuit, and a variable resistor connected to the waveform shaping circuit and the brightness adjustment switch. The triangular waveform generation circuit charges and discharges the capacitor to generate a triangular waveform signal. The waveform shaping circuit compares the triangular waveform signal with a reference potential to generate a duty signal. The variable resistor varies the resistance in accordance with the operated amount of the brightness adjustment switch to determine the reference potential supplied to the waveform shaping circuit.
The rheostat generates the duty signal at a high level or a low level based on the reference potential determined by the variable resistor. More specifically, the rheostat generates the duty signal at a low level when the potential of the triangular wave signal is greater than the reference potential and generates the duty signal at a high level when the potential of the triangular wave signal is less than the reference potential. In this manner, the rheostat provides the lamps and LEDs with the duty signal having a duty ratio that is in accordance with the operated amount of the brightness operation switch. The lamps and LEDs are illuminated with a brightness that is in accordance with the duty ratio of the duty signal provided from the rheostat. Normally, the lamps and LEDs become brighter as the duty ratio increases.
Referring to FIG. 1, the brightness-duty relationship of the lamps differs from that of the LEDs. In FIG. 1, the triangles (Δ) indicate the brightness-duty relationship of the lamps, and the crosses (x) indicate the brightness-duty relationship of the LEDs. As the duty ratio increases, the brightness of the lamps increases as if it is plotted along a parabolic curve, whereas the brightness of the LEDs increases in a linear manner. Accordingly, to equalize the brightness of the lamps with the brightness of the LEDs using the same brightness adjustment switch, the duty ratio of either one of the lamps and the LEDs must be adjusted.
To do so, two waveform shaping circuits, one for the lamps and the other for the LEDs, may be employed with a variable resistor connected to each waveform shaping circuit. In this case, the varied resistance of one of the two variable resistors relative to the operation amount of the brightness adjustment switch is adjusted. Accordingly, the two waveform shaping circuits output duty signals having different duty ratios. For example, when matching the brightness of the LEDs with the brightness of the lamps, the varied resistance of the variable resistor in the intermediate operation range of the brightness adjustment switch is decreased. As a result, in the intermediate operation range, a duty signal having a duty ratio that is less than normal is provided to the LEDs.
However, such variable resistors are relatively large. Furthermore, a vehicle light adjustment system including the above rheostat has two variable resistors, one for the lamps and the other for the LEDs. This enlarges the vehicle light adjustment system. In comparison to the triangular wave generation circuit and the waveform shaping circuit, the ratio of the area occupied by the variable resistors in the entire system is large. In other words, the two variable resistors enlarge the system. For this reason, there is a demand for minimizing the quantity of variable resistors.